The present invention relates to a method for etching molybdenum silicide in semiconductor processing.
Reactive ion etching processes are used to fabricate devices having submicron sized features, such as semiconductor integrated circuit chips. These processes are used to selectively etch a substrate, where portions of the substrate are protected by a patterned etch resistant "resist" material, such as photoresist or oxide hardmask. The resist protected portions form "features" on the substrate which become part of the integrated circuit being processed. Etching is effected by introducing a process gas into a chamber and generating a plasma in the chamber to create an etch gas from the process gas. The etch gas etches the substrate to create volatile etch byproduct compounds, which are then removed from the chamber.
Typically, the process gas is a mixture of gases such as for example a mixture of Cl.sub.2 or CCl.sub.4, O.sub.2, and an inert gas such as He or Ar. Often a chloroflurocarbon gas is added to the process gas. However, there are several problems with these process gases. One problem is that chloroflurocarbon gases are environmentally toxic. Thus, processes using these gases are subject to strict environmental regulations.
Another problem is that these gases chemically react with the resist on the substrate, and cause relatively thick residues or deposits to form on (i) the chamber walls, (ii) the sidewalls of the etched features, and (iii) the resist material. This deposit or residue layer can flake off and form particles that contaminate the wafers. The contaminant particles are not detected until the wafer is fully processed and can results in loss of the entire wafer at a cost of $5,000 or more.
Another problem with these gases is that they etch features having "reentrant" profiles. By reentrant profiles, it is meant that the sidewalls of the features are inwardly sloped, forming angles of less than 85.degree. with the substrate. Reentrant profiles are caused by isotropic etching or undercutting, which occurs when etching proceeds horizontally below the resist layer, instead of vertically through the uncoated portions. It is preferable to have features having vertical sidewalls with angles close to 90.degree., which occur when the process gas anisotropically etches the substrate and etching proceeds vertically through the uncoated portions of the substrate.
Typical reactive ion etching systems can also result in high profile microloading. High profile microloading occurs when the cross-sectional profile of the features vary as a function of the spacing between the features on the substrate. It is desirable to have an etching process that provides features with uniform cross-sections regardless of the distance between the features, or the density of features.
It is also desirable to obtain high etch rates and a high etching selectivity ratio for process efficacy. The etch selectivity ratio is the ratio of the MoSi.sub.x etch rate to the resist etch rate. A high selectivity ratio is desirable to avoid excessive etching of the resist layer that can result in etching of the substrate below the resist layer.
Accordingly, there is a need for a method for selectively etching molybdenum silicide on semiconductor substrates that minimizes deposits on chamber walls; provides substantially anisotropic etching; and provides reduced profile microloading. It is also desirable to obtain high etch rates and high substrate to resist etch selectivity ratio for process efficacy.
The present invention is directed to a method that satisfies these needs. The method allows substantially anisotropic etching of substrates, reduced residue deposition, reduced profile microloading, high etch rates and a high etch selectivity ratio.
The method of the present invention comprises selectively etching a substrate having a molybdenum silicide layer with a resist material on portions of the molybdenum silicide layer. The method comprises the steps of:
(a) placing a substrate into an etch zone;
(b) introducing a process gas comprising SF.sub.6 and HBr into the etch zone, the volumetric flow ratio of SF.sub.6 :HBr being from about 1:10 to about 1:1; and
(c) generating a plasma in the etch zone to form an etch gas from the process gas, wherein the etch gas selectively etches the molybdenum silicide layer on the substrate.
Preferably, the process gas further comprises an oxygen containing gas. Preferably, the volumetric flow ratio of SF.sub.6 :HBr:oxygen containing gas is selected so that the sidewalls of the etched features are smooth and form angles of at least about 85.degree. with the substrate. More preferably, the flow ratio is selected so that the molybdenum silicide to resist etching selectivity ratio is greater than about 0.6, and most preferably, the flow ratio is selected so that molybdenum silicide etch rate is greater than about 500 .ANG./minute.